A multitude of doors are used in aircraft. There are external doors which allow access from the outside to the passenger compartment, and there are freight loading doors. Any unintendedly open door leads to problems as a result of a decrease in pressure in the cabin, and to damage of the aircraft structure as a result of torn-off doors.
To prevent any unintended opening of doors, said doors are kept in the closed position by means of an expensive locking mechanism. Before takeoff it must be ensured that all locking mechanisms are in the position “locked and secured”. For this reason the current position of locking devices is monitored by proximity switches and is centrally evaluated and displayed.
Most of the time this type of proximity switch is designed as an inductance with a ferrite core, operated as a free-running oscillator. If a magnetic conductor (target), which can be affixed to the monitoring mechanics, approaches said proximity switch, the magnetic resistance of the arrangement changes, and so does the inductance, and thus the oscillator changes its oscillation behaviour so that a change in the position of the locking mechanisms of a door leaf can be recognised. The ferrite core inserted in the inductances is nowadays required to ensure adequate sensitivity and thus reliable detection of the open position of the door leaf. Ferrite cores are however heavy. This has a negative effect in the case of aircraft, where weight savings are very important.